The present invention refers to a method and a device for keeping constant the density of a printed color, such as with an offset press.
Regular intensity and density of printed colors is very important if good printing quality is to be insured, and various solutions have been proposed. Previously known devices check viscosity of the printing colors being used. If these colors do not satisfy the required viscosity for the printing quality desired, the printer modifies the proportions in the ink/solvent mixture to change the viscosity. This is done by means of a mixer, the valves of which are governed by a signal responsive to the displayed viscosity measured by means of a standard viscosimeter. However, between the moment the printing quality starts degrading and the moment the printer acknowledges the flaw, a large quantity of material to be printed goes on running through the machine. This poorly printed material represents, of course, an important loss.
The known devices calculate the requested viscosity value. The viscosimeter is then manually set to keep this value by valves bringing to the mixer the ink, on the one hand, and the solvent on the other hand. Thus the constant viscosity of the ink/solvent mixture is maintained during the whole printing operation. The viscosity of this mixture can vary for different reasons, such as by solvent evaporation, changes of temperature, ink or solvent absorption by the material to be printed, etc. When the printer notices an inadequate color density, he will manually increase or lower the viscosity of the ink/solvent mixture. It should also be noted that the thixotropic or rheotropic characteristics of the ink/solvent mixture influence the viscosity.
Although the known devices act on the viscosity of the ink/solvent mixture, they do not automatically register the results on the printed material, and they require a manual setting during the running of the machine. The main drawback of this is the impossibility to detect and immediately correct a density variation of a printed color. The "newspaper" printing machines, for instance, with very high running speeds reaching sometimes 700-800 m/min, undergo losses of the color density changes. This is not acceptable for other printing jobs.